Switching power supplies are widely used in electronic devices. Generally, in a switching power supply, an AC voltage is derived from the power network and rectified into an unregulated DC voltage by a rectifier bridge. The unregulated DC voltage is then converted into the signal as needed by a switching circuit.
However, along with the wide application of the switching power supplies, more and more harmonic current are injected into the power network. The injected harmonic current increases the energy loss and reduces the power factor of the system. It reduces not only the quality but also the reliability of the power network. It may even burn out the protection circuit and other apparatus. As a result, the harmonic component of the switching power supplies should be reduced and the power factor should be improved.
A prior power factor correction (PFC) method is getting the peak input current of the switching circuit to follow the input voltage of the switching circuit. To reduce the electromagnetic interference (EMI), an EMI filter is often coupled between the power network and the rectifier bridge. So the average input current of the switching circuit is equal to the current derived from the power network, which is the input current of the switching power supply. For switching circuits with continuous input current (such as Boost converters), the input current of the switching power supply will be sine wave and in phase with the AC voltage. The power factor is high and the harmonic component is small. However, for switching circuits with discontinuous input current (such as Buck, Buck-Boost or Flyback converters), the situation is totally different.
FIGS. 1 and 2 are waveforms of prior switching circuits with discontinuous input current, wherein Iin is the input current of the switching circuit, Ipk is the peak input current, CTRL is the control signal of the switching circuit, lave is the average input current. As shown in the figures, when the peak input current Ipk is regulated to be sine wave, the average input current lave is not sinusoidal. Thus the total harmonic distortion (THD) of the switching power supply is high and the power factor is limited.